Improve link error messages

Reverse the reversal of source/target in message errors and add link
source and target to the error given when a validator is missing on a
link.

lib/link/link.js

@@ -62,7 +62,7 @@ class Link {
 
 		let validator = this._validators.get(message.type);
 		if (!validator) {
-			throw new errors.InvalidMessage(`No validator for ${message.type}`);
+			throw new errors.InvalidMessage(`No validator for ${message.type} on ${this.source}-${this.target}`);
 		}
 
 		if (!validator(message)) {
@@ -180,7 +180,7 @@ class Link {
 	 */
 	async waitFor(type, data) {
 		if (!this._validators.has(type)) {
-			throw new Error(`no validator for ${type}`);
+			throw new Error(`No validator for ${type} on ${this.source}-${this.target}`);
 		}
 
 		return new Promise((resolve, reject) => {

lib/link/messages.js

@@ -82,7 +82,7 @@ class Request {
 		// Target side.  Note: Source and target is reversed here.
 		if (this.links.includes(`${link.target}-${link.source}`)) {
 			if (!handler) {
-				throw new Error(`Missing handler for ${this.requestType} on ${link.target}-${link.source} link`);
+				throw new Error(`Missing handler for ${this.requestType} on ${link.source}-${link.target} link`);
 			}
 
 			link.setHandler(this.requestType, message => {
@@ -361,7 +361,7 @@ class Event {
 		// Target side.  Note: Source and target is reversed here.
 		if (this.links.includes(`${link.target}-${link.source}`)) {
 			if (!handler) {
-				throw new Error(`Missing handler for ${this.eventType} on ${link.target}-${link.source} link`);
+				throw new Error(`Missing handler for ${this.eventType} on ${link.source}-${link.target} link`);
 			}
 
 			link.setHandler(this.eventType, message => {